The Senate Republican health care bill ran into serious trouble late Monday when key GOP senators indicated they may block the Obamacare overhaul from proceeding to a vote this week.

Political turmoil has been building over the bill for days. But GOP tensions burst open after the nonpartisan Congressional Budget Office reported that 22 million more Americans would lose insurance coverage under the plan, and out-of-pocket costs for many of those buying policies on the Affordable Care Act marketplace would rise.

Senate Majority Leader Mitch McConnell hoped to start procedural votes by Wednesday, and President Trump called key senators over the weekend as support splintered.

It's the same political dynamic that stalled the House Republican bill last month, as conservative and centrist factions wrestle for dominance. Conservatives want a more complete repeal of the Affordable Care Act, which they hope will lower premium costs, while centrists are trying to avoid leaving millions of Americans without health coverage.

"Senate bill doesn't fix ACA problems for rural Maine," tweeted Sen. Susan Collins (R-Maine). "I will vote no on mtp," she said, referring to the "motion to proceed" to the bill.

Conservative Sen. Mike Lee of Utah is also working to change the bill so he can vote yes on the procedural motion.

"We are not there yet," Lee's spokesman said.

Senators have bristled at what they viewed as McConnell's secretive and rushed process, and several other senators said they wanted more time before voting.

Sen. Ted Cruz of Texas was among those Republicans who shared concerns in weekend calls with Trump.

“We continue to make progress,” Cruz told reporters Monday, as Democrats, who oppose the bill, planned an almost-all-night protest session.

Cruz is part of the gang of four conservatives -- including Sen. Ron Johnson of Wisconsin, Sen. Mike Lee of Utah and Sen. Rand Paul of Kentucky -- who have said they cannot vote for the bill as is. Among the changes being pursued is one provision that would allow insurers to offer cheaper policies that do not meet ACA’s requirements and another to let consumers sock more money into Health Savings Accounts

“We can get there and I’m hopeful we will get there,” Cruz said. However, he declined to say if he would agree to Wednesday’s procedural vote.

Also hesitant to proceed was Nevada Sen. Dean Heller, who has strongly criticized undoing Obamacare's Medicaid expansion that has enabled some 200,000 people to gain coverage in his state.

J. Scott Applewhite / Associated Press

Sen. Susan Collins, R-Maine, speaks amid a crush of reporters after Republicans released their long-awaited bill to scuttle much of President Barack Obama's Affordable Care Act, at the Capitol in Washington, Thursday, June 22, 2017. She is one of four GOP senators to say they are opposed to it as written which could put the measure in immediate jeopardy.

Sen. Susan Collins, R-Maine, speaks amid a crush of reporters after Republicans released their long-awaited bill to scuttle much of President Barack Obama's Affordable Care Act, at the Capitol in Washington, Thursday, June 22, 2017. She is one of four GOP senators to say they are opposed to it as written which could put the measure in immediate jeopardy.

(J. Scott Applewhite / Associated Press)

Alaska Sen. Lisa Murkowski, for example, wanted changes to help residents in her geographically far-flung state where health care costs are particularly high.

Some senators, though, dismissed the budget analysis and said keeping the ACA would be worse.

"It’s clear the CBO cannot predict the purchasing patterns for millions of Americans," said Georgia Sen. David Perdue, a Trump ally, in a statement. "This bureaucratic analysis will do nothing to prevent Obamacare from failing."

Others are weighing their votes.

Republican Sen. Bob Corker, whose office is receiving thousands of daily calls, spent part of Monday on the phone with health officials in Tennessee as he assesses the fallout of 22 million more Americans without health care in his state.

“I kind of figured it was going to be a pretty big number,” said Corker, who remains undecided. “There’s a lot of incoming.”